“You can use ultrasound to visualize the needle through which you deliver stem cells to the heart. But once those cells leave the needle, you’ve lost track of them,” he said.

As a result, key questions go unanswered: Did the cells actually get to the heart wall? If they did, did they stay there, or did they diffuse away from the heart? If they got there and remained there, for how long did they stay alive? Did they replicate and develop into heart tissue?

Gammbhir’s lab has figured out a way to “mark” stem cells before infusing them into the heart, rendering them visible to standard ultrasound as they’re squeezed out of the needle.

The key was to invent an innovative imaging agent, in the form of nanoparticles whose diameters clustered just below one-third of a micron. The nanoparticles’ main ingredient, silica, shows up on ultrasound. The particles were also doped with the rare-earth element gadolinium, so they can also be observed using MRI.

When Gambhir and his associates infused these nanoparticle-loaded stem cells into the hearts of healthy mice, they were indeed able to monitor the cells via ultrasound after they left the needle tip, guide them to the targeted area of the heart wall, and still get a strong MRI signal from the cells two weeks later.

Stem-cell therapy for damaged hearts isn’t going to be cheap anytime soon. But a one-time delivery, if it worked, could replace a lifetime of constant drug administration.